Supercap lights your way in times of need

You won’t find [Antoine] stumbling around in the dark. He just finished working on this LED flashlight which draws power from a super-capacitor (translated). He realized that lighting a high-efficiency LED takes so little power that there are many benefits in play when deciding to move away from batteries. When compared to a super capacitor, batteries have a shorter life span, are heavier, and take up more space.

The biggest drawback of a super capacitor in this situation is the low voltage operation. The output will start at 2.7V and drop as the current is discharged. [Antoine] used one of our favorite simple circuits to overcome this issue, the Joule Thief. That circuit is commonly seen paired with an LED in order to boost input voltage to a usable level. That’s precisely what’s going on here.

The final hack in his circuit is the addition of that red LED which you can see in the middle of the board. This takes the place of a Zener diode and drops the charging voltage to a safe level. That indicator light will not come on until the cap is fully topped off. This way it tells you when the device is done charging.

That’s not wholly true, Li-ion batteries may be pushed aside by supercapacitors in the very near future. While the energy density of a Li-ion battery is 160 W*h/kg and the energy density of the type of EDLC shown here is 6 W*h/kg, MIT LEES has produced a super cap with an energy density of 85 W*h/kg and EEStor claimed to have produced energy densities up to 400 W*h/kg. EEStor seems to have evaporated from existence though with a statement from the company being declared imminent for the last 4 years so I’d severely doubt any of their claims. Still while EDLCs may not yet have the same energy storage as Li-ion batteries they are quickly encroaching on the territory. Additionally they are considerably cheaper and the Capa-bus in Shanghai has demonstrated they are ready to handle some of the large power requirements of today.

I’m sure someone who know more than I will correct me, but I thought that the big advantage of super caps was that they could be charged almost instantly (at least compared to any other battery.) In which case making it USB charging seems to be completely missing the point.

But I don’t really care. Powering a single LED is not an engineering challenge and the project has a few nifty features.

Since he’s provided the exact values, we can calculate that the average current will be 65 mA and the average voltage over the capacitor will be 1.35 volts, so, the charge time should be approximately 7 minutes.

That can’t be right, because the current through the capacitor is at maximum 5V/56Ohm = 89 mA, but when the capacitor has, say 2 volts already in it, the current is reduced to 3V/56Ohm = 53 mA.

At the beginning of the charge, there’s 0 volts over the capacitor, and therefore no power going into it. The efficiency of the charge starts from (arbitrarily close to) zero and picks up as the voltage of the cap increases, but that also decreases the available power because the voltage across the resistor drops which decreases the current, which slows down the charging.

The only way you can get the calculated 36 Joules into the capacitor in about a minute is by assuming a constant 5 volts x 89 mA = 0.445 Watt across the capacitor, which yields a charge time of 80 seconds, but this simply doesn’t happen.

He’s wrong, the zener is connected backwards. When polarized directly a zener behaves as a normal silicon diode which has a voltage loss around 0.6-0.7 volts, depending on the current that flows into it due to it’s non linearity. To get the zener effect you have to connect it backwards: cathode to the positive and anode to ground. think of a zener as two diodes connected in antiparallel.

Were he to tune the circuit to resonance by testing with a variable resistor instead of just using the 1k, he could lower current draw for the same lumens by increasing frequency… effectively making his supercap charge last longer. A bit of attention to the toriod as well… iron powder in the right band to make a high Q resonant circuit, and it can be improved even more:)

Lurker here. Maybe someone can enlighten me, but the semiconductors in use here have finite resistance, so wouldn’t the capacitor slowly discharge over time? It might be negligible, but some of the posters above make it sound like it’ll never go dead.

Only if the cell phone manufacturers would put in a extra LED to help light our way sometimes, often using the display isn’t enough. Call me spend thrift A 5 dollar bill, and a nickle buys a key fob light from Walmart, they seem to last me a year. For me it wouldn’t matter how much during the day I spend near a USB port, if I forget to take off the key ring, and charge it. Nice project, but stuff this inside a functional USB drive I’d really be impressed. One more less daily carry item, not that carry my portable apps drive all that often.

But when the battery dies after abusing the flashLED as your flashlight, you’re sure going to be waiting a lot longer to have that flashlight back. And now you’re out your phone for a time too!
‘Course, if you’re using that function of your handheld for long enough to drain it’s batteries, you’ve probably got other things to worry about anyhow.

You’re lucky. How about those über-modern phones that intentionally have no buttons, at least not very important ones, and require both hands plus eyes to operate? :( I was lucky, too, in buying a HTC TyTN just before they became unavailable, and still didn’t found a decent replacement for it.

The Joule Thief can operate to low voltage, but it is not very effective (: Maybe it would be much better idea to use some charge pump like ICL7660, because they can be much better in efficiency terms. Also, no inductors :D
The circuit is somewhat non-standard, it goes like that:
Pin 3 – supply –
Pin 8 – supply +
Pin 5 – LED cathode
Pin 2 – C1 +
Pin 4 – C1 –
LED anode to +
C1 is 47-200 uF cap needed for 7660 operation.
I cannot test it right now because I don’t have a supercap, would be happy if someone shares results. You can make a flasher as well – connect 10n cap across pin 7 and supply +.

Just a thought, since the red led seems to be the only “protection” from overcharge. Couldn’t you put a micro relay in there that activates instead of a light? That would open the circuit to prevent overcharge. Just a thought, I’m not much of an electronics hacker these days but I’m just thinking from my old rat-shack 500 project electronic kit days and it seems like a good way to cut the juice before popping the cap.

Hi, i was wondering how you would use one of those “caps” as a batterie? How would you get it to keep the energy stored till it is ready to use?… i have sooo many questions. I dont know much about all this but thats why im here is to learn how to build cool stuff like this. Thanks for what ever info you guys reply.